Abstract A pediatric vaccine against HIV would have a significant clinical impact, because more than 150,000 infants are infected with HIV every year globally, despite the availability of antiretroviral drugs to prevent mother-to- child transmission. In addition, a pediatric HIV vaccine that offers protection in infancy and durable protective immunity prior to sexual debut would significantly reduce adolescent HIV infections. Developing a pediatric HIV vaccine will require to overcome specific immunological challenges dues of limitations of the early life immune system including 1) a reduced ability to provide T-cell help, which results in poor somatic hypermutation of antibodies and inadequate antibody affinity, and 2) the need for several vaccine boosts to achieve durable immunity. Paradoxically, recent studies have demonstrated that children can develop neutralization breadth earlier than adults, suggesting that the early life immune system could be more amendable for the elicitation of this highly desirable response through vaccination. A leading strategy for elicitation of broad neutralizing antibody response is to immunize with native live HIV envelope trimers. Yet, despite advances in stabilization and production of native-like HIV-1 envelope trimers over the last decade, typical vaccination strategies with HIV-1 Envelope SOSIP trimer products have been disappointing in their ability to raise broad and potent virus- neutralizing activity. Novel vaccine platforms may be needed to improve the immunogenicity of Envelope SOSIP trimer vaccines. In this collaboration, we will develop an innovative nanofiber pediatric HIV vaccine comprised of a scaffolded CH505 SOSIP Env trimer and a synthetic T-Cell epitope (PADRE) self-assembled into supramolecular nanofibers. We hypothesize that the PADRE- nanofiber conjugated HIV-1 CH505 SOSIP trimer vaccine (P-Q11 CH505 trimer) will enhance the magnitude and potency of tier 2 virus neutralization responses in small animal and infant non-human primate (NHP) models, and will be protective against homologous SHIV challenge in an infant NHP challenge model. Our specific aims are: 1) Develop and assess the antigenicity the nanofiber-conjugated CH505 SOSIP trimer subunit vaccine; 2) Define the immunogenicity of the P-Q11 CH505 trimer vaccine in neonatal rabbits and infant rhesus macaques in comparison to that of CH505 SOSIP Env trimer alone (Go/No-Go endpoint); and 3) Determine the ability of the P-Q11 CH505 trimer vaccine to protect against low dose oral SHIV challenge in an infant nonhuman primate model of late postnatal transmission via breastfeeding. This novel pediatric HIV vaccine strategy could overcome the challenges of infant vaccination, while taking advantage of the immunologic and practical benefits of early life immunization for elicitation of protective immunity.